1. Field of the Invention
The present invention relates to a hyperspectral imager and more particularly to a grating imaging spectrometer that minimizes optical aberrations, occupies a small packaging volume, and enables versatile coupling to front-end collecting optics that image a scene onto an entrance slit.
2. Description of the Related Art
There are three problems in designing an imaging spectrometer where light in a slice of an image field passing through an entrance slit is to be diffracted by a grating parallel to the slit and imaged onto a detector array for display or recording with good spatial resolution parallel to the slit and good spectral resolution perpendicular to the slit:
1. Eliminating astigmatism over the spectrum on the image plane.
2. Removing field curvature from the spectrum focused onto the image plane.
3. Obtaining good spatial resolution of the entrance slit; this entails eliminating astigmatism at different field angles from points on the entrance slit.
The third problem is probably the most difficult to overcome. In conventional imaging spectrometers, the spectrometer size is increased until the astigmatism at different field angles is reduced to an acceptable level. This technique is effective because increasing the spectrometer size for a given slit size reduces the field angle through the spectrometer, leading to a reduction of field aberrations such as astigmatism. But for some applications, such as for aerospace instrumentation, a small design volume is important.
U.S. Pat. No. 5,880,834, issued to M. P. Chrisp, titled, xe2x80x9cConvex Diffraction Grating Imaging Spectrometerxe2x80x9d, discloses a 1:1 Offner mirror system for imaging off-axis objects that is modified by replacing a concave spherical primary mirror that is concentric with a convex secondary mirror with two concave spherical mirrors M1 and M2 of the same or different radii positioned with their respective distances d1 and d2 from a concentric convex spherical diffraction grating having its grooves parallel to the entrance slit of the spectrometer which replaces the convex secondary mirror. By adjusting their distances d1 and d2 and their respective angles of reflection xcex1 and xcex2, defined as the respective angles between their incident and reflected rays, all aberrations are corrected without the need to increase the spectrometer size for a given entrance slit size to reduce astigmatism, thus allowing the imaging spectrometer volume to be less for a given application than would be possible with conventional imaging spectrometers and still give excellent spatial and spectral imaging of the slit image spectra over the focal plane.
The Chrisp instrument is forced to use a complex optical system to compensate for the off-axis slit location. In addition, its optical performance typically degrades if the magnification through the system differs significantly from 1:1. Finally, the Offner configuration offers no good location for a cold stop when it is used with collecting optics. Such a stop provides a convenient position for baffling and, in the case of infrared applications, for locating a cold stop to decrease focal surface irradiance due to near field emission.
Prism-based instruments, while typically providing a small size advantage, suffer from non-linear dispersion, and, like any instrument containing refractive optics, their alignment and focus are temperature dependent.
The hyperspectral imager of the present invention includes, in a broad aspect, a diffraction grating, a collecting reflecting element and a reimaging system. The diffraction grating has an entrance slit formed at an entrance slit location therein. The entrance slit has a long dimension oriented in a y-direction. It transmits the radiation from a slice of an incoming scene image. A collecting reflecting element receives the transmitted radiation of the incoming scene image and reflects the transmitted radiation to a diffractive surface of the diffraction grating. Grooves on the diffractive surface are substantially parallel to the y-direction. A reimaging system receives radiation diffracted by the diffractive surface. The reimaging system produces a spectral image of the entrance slit at a focal surface. The spectral image provides a spectrum of radiation propagating through the entrance slit into the hyperspectral imager, such that the spectrum of radiation from a first region in the y-direction can be distinguished from the spectra of radiation from other regions in the y-direction.
Utilization of a diffraction grating having an entrance slit location therein enables the collecting reflecting element to operate substantially on-axis. This results in a minimization of optical aberrations, permits configurations with magnification differing substantially from 1:1, and, for many configurations including the preferred embodiment, provides a convenient location for a cold stop within the spectrograph. In addition, it facilitates the design and fabrication of instruments comprising optical and structural elements all fabricated from the same material, e.g., aluminum, beryllium, silicon carbide. This, in turn, permits the design and fabrication of instruments that are inherently athermal. Finally, it facilitates the design of compact instruments.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.